Add IOPS unit tests

Currently this doesn't do much except test timers, but it's good to have
as we expand our array of plugins

Change-Id: I34664fd25f736624836207938e63ec7d8128c5f4
Reviewed-on: http://review.couchbase.org/28135
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>